In red-, orange- and yellow-color light emitting diodes, there has been conventionally employed an epitaxial wafer in which an epitaxial layer of gallium arsenide phosphide GaAs.sub.z P.sub.1-z (where 0.ltoreq.z.ltoreq.1) is formed on a single-crystalline substrate of gallium phosphide GaP or gallium arsenide GaAs.
When a lattice mismatching between the single crystal of the substrate of gallium phosphide GaP or gallium arsenide GaAs and the GaAs.sub.z P.sub.1-z epitaxial layer having an alloy composition formed on the substrate is large, this causes a misfit dislocation to take place at its interface, whereby a stress caused by the lattice mismatching is relaxed. When this dislocation propagates a composition constant layer, however, this results in that the internal quantum efficiency of the light emitting diode drops.
In order to relax such a lattice mismatching, there has been conventionally employed such an arrangement that an alloy composition gradient layer having a gradually varied alloy composition is provided as an interface between a single-crystalline substrate and a composition constant layer. That is, within the alloy composition gradient layer, an alloy composition gradually ascends along a thickness direction of the layer from an alloy composition of the single-crystalline substrate to a predetermined alloy composition of the composition constant layer. However, this results in that a stress caused by the lattice mismatching gradually accumulates in the entire epitaxial layer and amounts to a larger stress. This leads to generation of a misfit dislocation in the composition constant layer, with the result of reduction in lifetime and reduction in luminance.
For this reason, it has been so far demanded to develop a novel epitaxial wafer which can solve two problems that a misfit dislocation is propagated to the composition constant layer and that a stress caused by the lattice mismatching is exerted on the entire epitaxial layer.
This invention has been made for the purpose of solving the aforementioned problems or defects in the prior art, and an object of this invention is to provide a compound semiconductor epitaxial wafer which is high in luminance, quality and productivity.